This layer, while not excavated and only visible in a very narrow section, appears to be a thick level of preparation laid down directly on top of an earlier basalt pavement left in situ (7259) in order to raise the roadway of the main thoroughfare for the construction of a new basalt-paved road surface (7019).
Sketch of SUs 7255, 7256, 7258, 7259, 7260.
photo from south